People holding yellow umbrellas in support of victims of the mid-April Sewol ferry disaster, carry boxes containing signatures of South Koreans petitioning for the enactment of a special law after the disaster, as they march towards the National Assembly

The driver of a South Korean businessman wanted over the sinking of a ferry that killed 304 in April turned himself in on Tuesday, potentially unlocking the mystery of the man's final days after the country's worst maritime disaster in 44 years.

Prosecutors in the port city of Incheon said the driver, Yang Hoe-jung, turned himself in at their office, which is leading the probe into the role of businessman Yoo Byung-un in the sinking of the ferry Sewol.

The structurally defective and heavily overloaded ferry capsized and sank on a routine journey on April 16, killing 304 people, 250 of them teenagers from the same school on a class field trip. Twelve of their teachers were also killed.

The driver Yang was the last among a group of people close to Yoo who had been wanted for allegedly helping him elude South Korea's biggest manhunt.

Yang is thought by authorities to have been with Yoo, the head of a family that ran a network of companies that included the ferry operator, in the days before his body was found by a farmer at an orchard on June 12.

Police only identified the badly decomposed body as that of Yoo last week, although an autopsy and other extensive testing failed to indicate how he died or came to be in the orchard, forensic experts have said.

Yoo was accused of a range of questionable activities that included embezzlement and negligence that prosecutors believe led to the ferry disaster.

A reward of 500 million won ($488,000) had been posted for information leading to his arrest, the largest possible amount under South Korean criminal law.

Yoo's wife, brother and oldest son have been arrested but his younger son remains at large and is believed to be in the United States.

A senior prosecutor has said efforts have been made to work with U.S. law enforcement authorities to capture Yoo Hyuck-ki, who was considered Yoo's heir-apparent.

The trial of 15 surviving crew members on Tuesday continued to hear testimony from some of the 75 teenagers who survived the disaster. They spoke of the heroic leadership of a classmate amid the chaos, rather than that of the crew or coastguard.

"Some boy came around handing out (the life vests)," one student told the Gwangju district court, which has moved temporarily to Ansan, south of Seoul, to accommodate the students.

"We were in the hallway, and someone asked, 'Who is willing to go up in the helicopter?' and we raised our hands and went," she said. "Some boy asked, the boy with the life vests."

Two helicopters pressed into the rescue effort were able to take off a few passengers who climbed on to the starboard side of the listing vessel.

The identity of the boy with life vests was not clear, but he appeared to have been among those rescued by the helicopters.

The court has ordered the students' names withheld.

The crew face charges ranging from homicide to negligence for abandoning the ship after telling passengers, including the students, to remain in their cabins.

Some of the surviving children who testified on Monday said there was little help from coast guard rescuers who arrived as they scrambled out of the sinking ferry, with many classmates still trapped inside.

Others testifying on Tuesday painted a similar scene of students helping one another as the ferry listed sharply and passengers and equipment were thrown around violently inside the cabin, while the crew repeated orders for them to stay put.

"Afterwards at the hospital, we met a grown-up passenger, who said the students obeyed the announcement like fools, while other people broke the windows to get out," one student said.